How to identify a keymap conflict?
Go to File > User Preferences > Input, set the dropdown menu next to the search field to Key-Binding and enter AltB. It should list all operators bound to a key. It includes addon keybindings.



Addons can also capture events in modal operators, but it's really rare that they consume certain events and pass on the rest. So it would be pretty obvious that there's something running in the background, because a lot of actions wouldn't work.